In the case where control is performed in a plant such as an oil plant, it is sometimes necessary to measure a difference value between pressures of a process fluid located on positions away from each other.
In this case, conventionally pressure gauges are provided on two measurement positions, measurement values (electric signals) of the two pressure gauges are transmitted to an arithmetic circuit, and a difference is calculated therein, so that a difference value is measured between pressures on the two measurement points.
In addition, a differential pressure gauge is provided to measure a difference value of pressures and a process fluid is guided into the differential pressure gauge from two measurement positions by using a pressure transmitting pipe (or connecting pipe) for guiding a process fluid, so that a difference value is measured between pressures on the two measurement points.
In such a method using a pressure transmitting pipe, measurement may be hampered by clogging in the pressure transmitting pipe and a process fluid may flow to the outside when the pressure transmitting pipe is broken.
For this reason, in some cases remote seals for filling an enclosed liquid such as silicone oil are used instead of a pressure transmitting pipe and pressures of a process fluid are transmitted from two measurement positions to a differential pressure gauge, so that a difference value is measured between pressures on the two measurement points.
However, in the case of the method where pressure gauges are provided on two measurement positions and a difference is calculated between the measurement values of the two pressure gauges by using an arithmetic circuit, it is necessary to provide two expensive pressure gauges, increasing the cost.
Meanwhile in the case of the method where a pressure transmitting pipe is used to guide a process fluid from two measurement positions to a differential pressure gauge, measurement may be hampered by clogging in the pressure transmitting pipe and a process fluid may flow to the outside when the pressure transmitting pipe is broken.
Meanwhile in the case where remote seals are used to transmit pressures of a process fluid from two measurement positions to a differential pressure gauge, there is no probability that clogging may occur in a pressure transmitting pipe and the process fluid may flow to the outside. However, since the two remote seals are disposed in different environments, the influence of temperatures and so on may reduce measurement accuracy. Besides, this method may cause leakage of an enclosed liquid to the outside when the remote seals are broken.
An object of the present invention is to establish, in such a background art, another technique for measuring a physical parameter whereby a difference value between physical parameters on positions away from each other can be correctly measured without being affected by the ambient environment.
Another object of the present invention is to establish, for the above measurement, another technique for measuring a physical parameter whereby a difference value between physical parameters on positions away from each other can be correctly measured without being affected by a physical parameter not to be measured.